My old 31cm Dobsonian

My visual telescope(71Kb)

My portable telescope -
a 31cm f/6·5 Newtonian
on a Dobsonian Mount

31cm Dobsonian (145Kb)

When my 50-cm telescope replaced my 31-cm one (above, left) in the dome, I re-built it to make it portable (above, right). I didn't have much need for a portable telescope, but I decided to do it anyway. Having made a reasonable job of it, I decided that I really didn't need a portable telescope this size at all, so I sold it. This page remains as a memorial to my old, much used telescope.

The primary optics are on a full-thickness Duran 50 blank, hand ground and polished and brought to a good paraboloid, figuring it to a null against a 40cm flat. It has a 54mm (2·14-inch) enhanced-aluminium coated secondary. A fan underneath the box provides forced cooling of the thick mirror.

The original mount was heavy. It was my first of the Dobsonian design. It was made from "Craftwood", a dense reconstituted timber (MDF) product which is easy to work with, very strong, but results in a rather heavy product. It was not a problem as it was never taken anywhere. It had a quite high centre of gravity due to the heavy cardboard tube. The altitude bearings were plastic sewer-pipe fittings, 250mm in diameter. Straight teflon-on-formica forms the azimuth bearing, but another ring of teflon in the centre takes most of the weight, resulting in nice smooth motion. I built the mounting in a weekend. Saturday morning I had some sheets of wood, Sunday evening I was observing. Such is the beauty of this design (although I'd rather have an equatorial any day!).

I made Dave Lane's MicroGuider III encoder interface system and so now push my telescope around while looking at a computer screen of the sky. Dave's ECU software integrates beautifully but I use Bill Gray's Guide software because it can incorporate local pointing corrections for greater accuracy. I used US Digital's 8192 counts/revolution encoders.

Encoders are fitted to the telescope. Using the pointing modelling facility in Guide I can centre the telescope to within a few arcminutes of any point in the sky.

The best thing I ever did was to make the adjustable seat that is shown in the upper-left picture. Seated is the most comfortable way to observe. My seat allows you to position your body so that the eye is at exactly the right height for the eyepiece. No awkward ladders for me!

Update...2006 January

The new mount uses some of the old bits but is mostly new. The re-build was intended to make it portable, plus allow use of my new binoviewer. This meant that the focal surface had to be out far further than I would normally place it.

The top end is nothing special, but this picture shows the new focuser I have. I found it on eBay and is made by a place calling themselves "wyorock" - see their web page for more information. It's very nice and the very low profile helps in allowing the binoviewer to come to focus.

The truss is attached as a single pieces. The individual poles are 25mm RHS aluminium, joined to pieces of 25mm angle. The bottom truss attaches to the base like this, while it attaches to the top end thus. I may replace the wingnut with a custom knob, but it works well enough.

Azimuth encoder + MGIII Altitude encoder


home back to ATM


Page last updated 2007/21/15
Steven Lee